Think im being stupid with this one but ill ask in anycase:

I have a descent down a hill measured at -2. I need to find the angle of the descent so i use the following formula:

/a//b/cos(theta)

so rearranging :

arccos a . b / /a//b/

where a = -2 and b is the unit vector i. Right when Iam working this out i get pi which is ridiculous as this means the descent is vertical!!! clearly im working something out wrong but not sure what it is.

(Note : I am learning this stuff in my spare time - i am firefighter full time - so if this seems really basic to some i do apolgise)